wanted to update a Base E169 stick (already debranded) to FW 11.314.07.00.52 (downloaded from/via 3g-modem.wetpaint.com) and higher, but did not succeed. Hardware CD57TCPU.

Original problem was that the speed shown never came over 384kBit/s in all networks I used and whereever I was.
1st of all not all USB connectors liked the stick. Then XP detected the device, but not the Mobile Partner.

When I ran the update wizard, an error message came "Terminal plugged not firmware, the wizard can work on - update failure" or "IMEI error".

After that process the Mobile Partner Diagnostics shows the new FW version, but this changes after some time (plug/unplug/hibernate/launch again) automatically to the old version.

I tried with another XP machine, there the update wizard asked for a 8-digit PIN ("Firmware not compatible to installed version - Please enter PIN").

I plugged to a Vista laptop, nothing happened. An E220 was detected flawlessly.

I tried with another XP machine, there the update wizard asked for a 8-digit PIN ("Firmware not compatible to installed version - Please enter PIN").

Have you run that update in the end? If not, you can calculate the flashing code at Online Huawei Modem Unlock Calculator and then retry that update.
I guess something is wrong with the firmware, so reflashing it might help.

But the easiest method to enter AT-commands probably is installing MWconn, a small generic dashboard, through which you can enter AT-commands in the "Servicekonsole" under the "Gerät" tab, as shown below:

I tried, with PuTTY no problem.
All my 3 modems (E220, E169, E160) do not accept even one command except atz and ati.
I sent to both ports, modem port (here: COM8 ) and 3G PC UI Interface port (here: COM9).

E220 and E169 show COMMAND NOT SUPPORT, the E160 ERRRROR endlessly.
After some time the E220 showed ^RSSI:28 (signal strength).

???

Are there also hardwaremods known (esp. E220) to disable the flashdrive?